[Rust] [DataFusion] Implement extensible configuration mechanism

[asfimport]

We are getting to the point where there are multiple settings we could add to operators to fine-tune performance. Custom operators provided by crates that extend DataFusion may also need this capability.

I propose that we add support for key-value configuration options so that we don't need to plumb through each new configuration setting that we add.

For example. I am about to start on a "coalesce batches" operator and I would like a setting such as "coalesce.batch.size".

For built-in settings like this we can provide information such as documentation and default values and generate documentation from this.

For example, here is how Spark defines configs:

  val PARQUET_VECTORIZED_READER_ENABLED =
            buildConf("spark.sql.parquet.enableVectorizedReader")
              .doc("Enables vectorized parquet decoding.")
              .version("2.0.0")
              .booleanConf
              .createWithDefault(true) 

Reporter: Andy Grove / @andygrove
Assignee: Andy Grove / @andygrove

_{Note: This issue was originally created as ARROW-11059. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Migrated to github: apache/datafusion#138